Asthma is an increasingly common disease that remains poorly understood and difficult to manage. There is a fundamental gap in understanding how environmental factors acting in concert with genetic predisposition leading to the preferential development and activation of Th2 cells by allergens in asthma and allergic individuals. Our long term goal is to better understand the initiation and maintenance of allergen-specific Th2 responses, and to identify potential therapeutic targets for treatment and prevention of allergic diseases. The overall objective of this application is to determine the mechanism of TSLP-mediated Th2 sensitization and Th2 memory. TSLP is critical in the pathogenesis of asthma and atopic dermatitis as studies in our lab demonstrated that mice deficient in TSLP receptor is greatly impaired in allergic inflammation while others established the importance of TSLP in the pathogenesis of human asthma and atopic dermatitis. Based upon our preliminary study, our central hypothesis is that TSLP produced by dendritic cells and mucosal epithelial cells acts on DCs and CD4+ T cells to breakdown airway mucosal tolerance, initiate and maintain Th2 immunity and is thus a key regulator of allergic diseases. We will test the hypothesis by pursuing three Specific Aims: 1) Define the role of TSLP in the breakdown of airway tolerance; 2) Define the role of dendritic cell produced TSLP in Th2 polarization; and 3) Define the role of TSLP in generation and maintenance of Th2 memory cells. In the first aim, we will study how TSLP suppresses antigen induced regulatory T cells and promotes Th2 sensitization against inhaled harmless antigens. In the second aim, we will use an already proven retrovirus mediated gene knockdown and overexpression in bone marrow derived dendritic cells to study role of TSLP in Th2 sensitization during antigen priming. In the third aim, we will adoptively transfer in vitro polarized Th2 cells to study how TSLP affects the generation and maintenance of Th2 memory. We will also test whether TSLP blockade is able to dampen already established antigen-specific Th2 memory, a study with clinical significance. None of these questions have been explored. Our proposed research to explore TSLP-mediated breakdown of airway tolerance, initiation of Th2 sensitization, and maintenance of Th2 memory will lead to a better understanding of the pathogenesis of human allergic diseases. The results of this study are expected to have an important positive impact not only on the scientific understanding of the etiology of allergic disorders but also on the development of new interventions for the prevention and/or treatment of allergic diseases.